1. Field of the Invention
This invention relates generally to optical telecommunication systems and, more particularly, to transceivers comprising photonic integrated circuits employed in such systems.
2. Description of the Related Art
Wavelength-division multiplexed (WDM) optical communication systems are known in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber. Such systems typically include transmitters at an ingress of a network infrastructure of the optical communication system and receivers at an egress of the network infrastructure. Exemplary transmitters may include a plurality of signal channels, each including a laser associated with a particular wavelength, a modulator configured to modulate the output of the laser to provide a modulated signal, and an optical combiner to combine each of the modulated signals into a multiplexed output for delivery over the network infrastructure of the optical communication system. Exemplary receivers may include a demultiplexer to demultiplex the received multiplexed output, as well as other components which convert the received modulated signals into corresponding signals in the electrical domain.
Conventionally, WDM systems have been constructed from discrete components. For example, the lasers, modulators, combiners, and decombiners have be packaged separately and provided on a printed circuit board. More recently, however, many WDM components have been integrated onto a single chip, also referred to a photonic integrated circuit (PIC).
In order to further increase the data rates associated with WDM systems, various modulation formats have been proposed for generating the modulated laser output. One such optical signal modulation format, known as polarization multiplexed differential quadrature phase-shift keying (“Pol Mux DQPSK”), can provide spectral densities with higher data rates per unit of fiber bandwidth than other modulation formats, such as on-off keying (OOK).
However, systems designed to transmit and receive Pol Mux DQPSK signals, however, typically have additional components, both active and passive, and greater complexity than transmitters and receivers designed for use with other modulating techniques.
Additionally, some systems which include coherent receivers designed to receive polarization multiplexed signals, such as Pol Mux DQPSK or Pol Mux QPSK signals for example, may require a local oscillator. The local oscillator is used in the demodulation process, the phase of the local oscillator being compared to a phase of the incoming signal, a Pol Mux DQPSK signal for example, the change in phase indicative of the data received.
Thus, there is a need to integrate components associated with a PM DQPSK transmitter and receiver, referred herein as a transceiver, on a single photonic integrated circuit in order to realize a WDM system transceiver having few discrete components resulting in a compact design leading to reduced costs.